This invention relates to turbine engines, and, more particularly, to the reduction of frictionally induced wear damage within the rotors of the compressor and fan stages of these engines.
When two pieces of material rub or slide against each other in a repetitive manner, the resulting frictional forces can cause damage to the materials through the generation of heat or through a variety of fatigue processes generally termed fretting. Some materials systems, such as titanium contacting titanium, are particularly susceptible to such damage.
When two pieces of the same or substantially the same metal, for example titanium, are rubbed against each other with an applied normal force, the pieces can exhibit another type of surface damage called galling. Titanium may gall after as little as a hundred cycles.
Both fretting and galling increase with the number of cycles and can eventually lead to failure of either or both pieces by fatigue.
The use of titanium parts that can potentially rub against each other occurs in several aerospace applications. Titanium alloys are used in aircraft and aircraft engines because of their good strength, low density and favorable environmental properties at low and moderate temperatures. If a particular design requires titanium pieces to rub against each other, the types of damage just outlined may occur.
In one type of aircraft engine design, a titanium compressor disk, also referred to as a rotor, or fan disk or rotor has an array of dovetail slots in its outer periphery. The dovetailed base of a titanium compressor or fan blade fits into each dovetail slot of the disk. When the disk is at rest, the dovetail of the blade is retained within the slot. When the engine is operating, centrifugal force induces the blade to move radially outward. The sides of the blade dovetail slide against the sloping sides of the dovetail slot of the disk, producing relative motion between the blade and the rotor disk.
This sliding movement occurs between the rotor and blade titanium pieces during transient operating conditions such as engine startup, power-up (takeoff), power-down, and shutdown. With repeated cycles of operation, the sliding movement can affect surface topography and lead to a reduction in fatigue capability of the mating titanium pieces. During such operating conditions, normal and sliding forces exerted on the rotor in the vicinity of the dovetail slot can lead to galling, followed by the initiation and propagation of fatigue cracks in the disk. It is difficult to predict when initiation of cracks may occur or extent of damage in relation to the actual number of engine cycles. Engine operators, such as the airlines, must therefore inspect the interior surfaces of the rotor dovetail slots frequently, which is a highly laborious process.
Various techniques have been tried to avoid or reduce the damage produced by the frictional movement between the titanium blade dovetail and the dovetail slot of the titanium rotor disk. At the present time, the most widely accepted technique is to coat the contacting regions of the titanium pieces with a metallic alloy to protect the titanium parts from fretting or galling. The sliding contact between the two coated contacting regions is lubricated with a solid dry film lubricant containing primarily molybdenum disulfide, to further reduce friction.
While this approach can be effective in reducing the incidence of fretting or fatigue damage in rotor/blade pieces, the service life of the coating has been shown to vary considerably. Furthermore, the application process for applying the metallic alloy to the disk and the blade pieces has been shown to be capable of reducing the fatigue capability of the coated pieces. There exists a continuing for an improved approach to reducing such damage and ensure component integrity. Such an approach would desirably avoid a major redesign of the rotor and blades, which have been optimized over a period of years, while increasing the life of the titanium components and the time between required inspections. The present invention fulfills this need, and further provides related advantages.
A new approach to reduce the incidence of fretting in high temperature components described in European Patent Application 89106921.3 utilizes two independent, but superposed foils having material contact surfaces with a low coefficient of friction, but surfaces which mate with the dovetail and dovetail slot having high coefficients of friction. The foils allow sliding movement along the material contact surfaces, having the low coefficient of friction, but prevent sliding between the foil and the mating parts due to the high coefficient of friction. Experience with this type of design has shown that each of the thin foils gradually works its way out of the dovetail slot region, leaving the blade dovetail and rotor dovetail slot in contact, resulting in fretting. In one embodiment, the foils have formed flanges. The flanges are necessarily small because of the small gap between blade dovetail and the rotor dovetail slot, and although providing some improvement, are not expected to eliminate the problem of gradual movement of the foil.
In another new approach described by Herzner et al. (application Ser. No. 641,230, filed Jan. 15, 1991) a reinforced shim is attached to the dovetail of a fan or compressor blade, and the blade with attached shim is placed in the rotor dovetail slot. The shim is reinforced with a metallic doubler, configured in such a way as to prevent the shim from working its way out of the dovetail slot region. The shim is made from a material other than the titanium alloys frequently selected for compressor and fan rotors and blades, and phosphor bronze is identified as the preferred material for the portion of the shim which is interposed between the contacting surfaces of the blade and rotor.